1. Field of the Invention
The present invention relates generally to an electro-photographic printer or copier, and, in particular, to an apparatus for exposing a photoconductive intermediate image carrier of the printer or copier with different gray scale levels using a light source.
2. Description of the Related Art
Electrographic printing or copying devices are known (as shown for example in European patent document EP 0 683 954 B1). Such a printing device contains a driven intermediate support, which may be a photo-conducting drum, on which charge images of images to be printed are generated. The charge images have toner applied to them, the toner is transferred to a print medium, such as paper, and the toner is fused onto the print medium. To accomplish this, the printing device contains the following components grouped around the intermediate support: an exposure device (which is a character generator), a developer station, a transfer station, a cleaning station and a charge device. The charge device is used to charge the intermediate support to, for example, 500 Volts and then for generation of the charge images of the images to be printed with the exposure device to, for example, approximately 70 Volts. The charge images then have toner applied to them in the usual manner in the developer station. The transition from toner to the intermediate support however only takes place if sufficient current exists between the developer station, specifically the developer roller, and the discharged regions of the intermediate support. For instance, if the developer roller has a potential of 220 Volts and the charge images on the intermediate support have a potential of about 70 Volts, then a field results, which takes the toner from the developer station to the intermediate support. The potential must be reduced at least by means of a discharge of the intermediate support in order to make it possible to ink the intermediate support. This reduction in potential will be referred to in the following as a development threshold or development level. In the transfer station, the toner images are then transferred to the print medium. Finally, in the cleaning station the intermediate support is cleaned of residual toner. With that a new print operation can start.
From European patent document EP 0 683 954 B1, which is incorporated herein by reference, it is known to use LEDs (light emitting diodes) as an exposure device. Generally, these are arranged in a comb of adjacent LEDs, the comb lying at a right angle to the moving direction of the intermediate support. The image to be printed is broken down into rows and columns of raster cells (also referred to as macro-pixels), whereby the raster cells are further divided into output pixels (micro-pixels) and an LED is assigned to each output pixel. In order to generate raster cells with different gray levels, individual output pixels of the raster cell can be exposed accordingly. In order to further increase the number of gray level steps, the LEDs can be used in a multi-level operation, i.e. the emitted light energy can be changed by means of variation of the turn-on time and/or of the field current of the LEDs. A realization of such a multi-level operation is described in the above cited European patent document EP 0 683 954 B1.
European patent document EP 0 663 760 A1 also describes a digital LED printer with an LED comb, in which printed images of different gray levels are generated by modulating the exposure time of the LEDs.
A further arrangement with which an LED comb can be controlled is disclosed in U.S. Pat. No. 4,750,010. Once again the gray level of a raster cell of the printed image is set by the duration of the light pulses of the LED allocated to the raster cell.
U.S. Pat. No. 5,255,013 describes an LED printer head which provides one LED per raster cell (pixel) from a comb of LEDs and in which the gray level of the image to be printed that has been divided into raster cells is generated per raster cell by means of a series of successive rapid light pulses of the allocated LED.
In European patent document EP 1 018 834 A2 a character generator with LEDs is described, in which the light energy of the individual LEDs is set up in such a way that a desired dot can be generated on an intermediate support and the dot printed on the print material. In the process the fact is taken into consideration that the discharge curves of adjacent dots mutually influence each other. The light energy of the LED generating the dot on the intermediate support is set up in such a way that it does not become too great and does not discharge the intermediate support too strongly. The light energy is determined in the process by the light emission per LED and per time. To carry out this method, the light energies to be emitted for each LED (and each dot) must be calculated separately, which requires a time-consuming activation.
European patent document EP 0 564 868 A2 described a character generator which uses a laser to generate images on an intermediate support. Differing gray levels of the individual pixels in a row direction are generated as a result of activating the laser with an appropriate pulse width signal. To achieve a sharp edge in the transition on the border of an image, the pulse width signals fed to the laser of the pixels on the border are shifted in the direction of the nearest pixel within the image. Since the laser is fed with pulse width signals of variable width per pixel, the laser emits variable light energies per pixel.
European patent document EP 0 663 760 A1 describes an LED printer which exhibits a control architecture, with which a multitude of gray levels can be generated. To achieve a light emission for the LEDs that is independent of the properties of the individual LEDs, the characteristic properties of the individual LEDs are measured, a correction signal is taken from this information which is then fed to the activation circuit for the LEDs and is combined with the activation signal for the LEDs.
European patent document EP 0 388 833 A2 discloses an additional character generator with LEDs. Here a correction of the activation signals for the LEDs is also made depending on the environmental conditions.
U.S. Pat. No. 6,280,012 B1 describes a character generator for ink-jet heads. To avoid an overloading of the character generator for the case that a number of ink-jet heads are active at the same time, the ink-jet heads are combined into groups and activated in a staggered fashion.
U.S. Pat. No. 4,661,859 describes a printer with which gray levels can be printed. The data representing the gray level per pixel are fed to a counter, which correspondingly increments the value of the data. The counter status is converted into a pulse length signal, which is used for activation of a laser.